1. Field of the Invention
This invention relates generally to the production of crystalline ribbons on a continuous basis and more particularly is directed towards a novel method and associated apparatus for removing heat from the crystal growing surface area of a pool of molten crystalline material from which a crystalline ribbon is pulled in a generally horizontal direction.
2. Description of the Prior Art
In co-pending application Ser. No. 051,998 filed June 25, 1979, entitled "Method and Apparatus For Producing Crystalline Ribbons", there is disclosed a method and associated apparatus for pulling a ribbon of crystalline material such as silicon from a melt of the material, the ribbon being pulled in a generally horizontal direction from the surface of the melt. In the application there is disclosed a thermal impedance disposed below the surface of the melt to provide stability in the formation of the ribbon and to provide the proper temperature gradients conductive to the efficient formation of the ribbon from the molten materials.
It has been the practice to form single crystal semiconductive wafers for various electronic applications such as solar cells etc. by pulling a seed crystal slowly upwards in a generally vertical direction from a pool of molten silicon. In this technique a crystalline rod is grown which is then sliced transversly into thin wafers which may be further cut into smaller components. The technique is both slow and expensive in view of the waste involved in the various cutting operation. Considerable effort has been made in an attempt to form the semiconductive material from the melt directly into a ribbon in order to eliminate the waste and time involved in cutting rods into wafers. A thin, monocrystalline ribbon, once formed, may then be suitably scored into the desired pattern and several elements broken away from the ribbon for component use.
While the technique of drawing a ribbon directly from the melt has successfully produced single crystal ribbons, there still exist problems with respect to controlling the speed of crystal formation and the quality of the ribbon, particularly with respect to the uniformity of dimensions and operating stability of the system.
Accordingly, it is an object of the present invention to provide improvements in the method and associated apparatus for directly forming ribbons of crystalline material on a continuous basis.
Another object of this invention is to provide a novel method and associated apparatus for producing high quality ribbons of crystalline material such as silicon or the like at a high rate of production and of uniform dimensions.
A further object of this invention is to provide a novel means for removing heat at a controlled rate from the surface of the melt to enhance crystal formation.